Radio Frequency Identification (RFID) is widely used in many fields, such as industrial automation, commercial automation, and transportation control management, and so on. In general, a microstrip antenna itself has the shortcoming of a narrow frequency to cause many restrictions on an antenna design. Most are designed into a single frequency and a narrow frequency. UHF electronic tags often use printed antennas. The majority are microstrip antennas and printed dipoles. If a UHF electronic tag is applied to a license plate, the metallic surface of the license plate makes the signal reflected easily. This may lead to an abnormal signal. Sometimes, the RFID tag cannot be activated and read. A conventional electronic license plate is a combination of an electronic tag and a metallic license plate. The electronic tag is mounted in a slot of the license plate. Although the problem that the metallic surface of the license plate affects the antenna of the electronic tag has been improved and the performances of long distance reading and transmitting signals can be achieved, it still has the problem that the electronic tag mounted in the slot may disengage from the slot. In particular, mounting an electronic license plate is not a policy of the traffic policy. The owner of a vehicle with an unauthorized electronic license plate may be punished because of modification of the license plate. A radio frequency identifier structure mounted to a license plate is commercially available. An outer side of a metal plate integrally formed with an antenna is provided with a slot. An electronic tag is embedded in the slot. The license plate with the electronic tag is mounted to a vehicle as radio frequency identification. Although this way neither modify the license plate and nor transgress against the low, it is a complicated process for the metal plate integrally formed with the antenna to be formed with the slot. Besides, after the electronic tag is placed in the slot, the slot is filled with glue for positioning the electronic tag, and the surface of the glue needs polishing and coating (or painting) and other surface treatment process, resulting in low production capacity (low production efficiency) and high manufacturing costs. Sometimes, due to economic considerations, it affects the popularity of mounting RFID tags to vehicles. Accordingly, the inventor of the present invention has devoted himself based on his many years of practical experiences to solve these problems.